Method of hardening steel plates

ABSTRACT

THIS INVENTION RELATES TO A METHOD OF HARDENING STEEL, AND MORE SPECIFICALLY TO A METHOD OF HARDENING QUENCH HARDENABLE STEEL PLATES AS THEY ARE BEING MOVED THROUGH A QUENCH ZONE. THE IMPROVED QUENCH IS CHARACTERIZED BY A NEW COMBINATION OF THE RATE OF WATER APPLICATION, WATER PRESSURE, THE ANGLE OF WATER IMPINGEMENT UPON THE SURFACE OF THE PLATE, AND THE AREA OF DIRECT IMPINGEMENT, ALL IN A PARTICULAR RELATIONSHIP. THE QUENCH WATER DIRECTLY IMPINGES BOTH SIDES OF A PLATE BEING HARDENED, THROUGHOUT THE COMPLETE AREA OF THE QUENCH ZONE SO AS TO COOL THE STEEL AT THE   MAXIMUM POSSIBLE RATE AND MINIMIZE THE FORMATION OF HIGH TEMPERATURE TRANSFORMATION PRODUCTS.

Se t. 4, 1973 G. F. MELLOY ETAL 3,756,869

METHOD OF HARDENING STEEL PLATES Filed Sept. 3, 1970 3 Sheets-Sheet 1 Fig.l

4O 7O .IOO 200 300 400 600 800 I000 QIFsin e INVENTORS George F. Melloy Joseph W. Hlinko Sept. 4, 1973 YIELD STRENGTH Filed Sept. 3,- 1970 G. F. MELLOY ET AL 3,756,869

METHOD OF HARDENING STEEL PLATES 3 Sheets-Sheet .3

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n00 I y no f I000 0 0 I 900 I 2 .)V-|6 I00 800 o 700 I: I8 I I 90 600 I H II I 0 0 h 9 500 0 a0 u I 400 I I! J y I a" x I 0 70 200 I I |o0 ,0 6O I O Q U sin e A INVENTORS George Mel'loy Joseph W. Hlinka Sept. 4, 1973 G. F. MELLOY ETAL METHOD OF HARDENING STEEL PLATES 3 Sheets-Sheet 3 Filed Sept. 5, 1970 INVENTORS George E Mel/0y Joseph W H/in/ra MW Hil United States Patent O Int. Cl. C21d 9/46, 1/62 US. Cl. 148-143 1 Claim ABSTRACT OF THE DISCLOSURE This invention relates to a method of hardening steel, and more specifically to a method of hardening quench hardenable steel plates as they are being moved through a quench zone. The improved quench is characterized by a new combination of the rate of water application, water pressure, the angle of water impingement upon the surface of the plate, and the area of direct impingement, all in a particular relationship. The quench water directly impinges both sides of a plate being hardened, throughout the complete area of the quench zone so as to cool the steel at the maximum possible rate and minimize the formation of high temperature transformation products.

CROSS-REFERENCES TO RELATED APPLICATIONS This application is a continuation-in-part of copending application Ser. No. 563,016, filed June 22, 1966, now abandoned, which is a continuation-in-part of application Ser. No. 542,982, filed Mar. 14, 1966, now abandoned, which is a continuation-in-part of application Ser. No. 509,593, filed Nov. 24, 1965, now abandoned.

BACKGROUND OF THE INVENTION It is well known to harden steel by heating it to a temperature at which the steel is austenitic and then cooling it from that temperature by an agitated submerged water quench. This is a practical method for hardening small parts, but it is not readily adapted to the hardening of large plates because of the difiiculty of providing uniform agitation throughout a large tank and because nonuniform agitation results in erratic and non-uniform physical properties. Water sprays and jet streams of water have been used in the past to provide quenching of steel plates but these have produced effective hardening only with relatively hardenable alloy steels or with relatively thin plates of carbon steel.

PRIOR ART Heretofore, adequate quenching of relatively thick steel plates has required such inordinately large amounts of cooling water as to present insurmountable problems in dealing with such amounts. As a practical matter, recourse had to be made to the inclusion of alloys in the steel to obtain the desired hardening properties.

The prior art as exemplified by Huseby US. 3,294,599 illustrates quenching with large amounts of water under high pressure in the form of discrete continuous rod-like jet streams. 7

However, Huseby does not show any appreciation for the difference in quench effectiveness resulting from gaps between areas which are directly impinged by his rod-like jet streams, and fails either to disclose specific values for the rate at which quench water is supplied or to relate the quantity of quench water with the area directly impinged by the quench water.

Clumpner et al., US. 3,300,198 is directed to progressive quenching of non-ferrous metals by a sequence of Patented Sept. 4, 1973 discrete water sprays of decreasing intensity. The Clumpner patent discloses water spray quenches having a pressure as high as 350 p.s.i. and water flow rates as high as one gallon per minute per square inch of effective impact area. However, Clumpner et al. disclose distinct gaps between the discrete areas being directly impinged by the sprays and fail to indicate any appreciation of the existence of any maximum rate of heat removal during quenching. Furthermore, although the Clumpner et al. quench may be suitable for non-ferrous alloys the presence of gaps between areas being directly impinged by the sprays would not result in uniform hardening of steel plates having a thickness approaching the limiting thickness for a particular hardenability.

The Scott patent, US. 2,776,230, discloses progressive quenching of steel pipe by means of overlapping water sprays having a pressure as high as 100 p.s.i. Scott also discloses correlating several factors characteristic of his spray which are designated as 1) angle of divergence, (2) mean free path, (3) angle of incidence and (4) overlap of area of impingement. However, Scott fails to provide any indication of the rate at which he supplies water or the area impinged by his water sprays.

The prior art illustrated by the aforementioned patents fails to disclose or to suggest evaluating quench effectiveness by means of applicants combination invention, hereinafter more fully described. In addition there is no suggestion in the prior art of the existence of a critical value for removal of heat at a maximum rate.

SUMMARY OF THE INVENTION Applicants have discovered an improved water quench for steel. plates which removes heat at a maximum rate by combining in a particular manner, certain characteristics, i.e., the rate at which water is supplied, the pressure of the water, the angle at which the water is impinged upon the plate, and the area of the surface being impinged. This quench method is particularly useful for hardening of quench hardenable steel plates being moved through a quench zone at a rate which will minimize the formation of high temperature transformation products. Applicants have discovered that steel plates can be effectively hardened according to their method in thicknesses greater than, and/or with hardenability less than, those previously possible by water quenching, providing the aforementioned quench characteristics are maintained at or above certain limits and bear a certain relationship to each other, as hereinafter more fully described.

The principal object of this invention is to provide an improved method of hardening steel plates by means of a water quench.

Another object is to provide an improved method of quenching'steel plates by directly impinging a spray or sheet of water completely across a zone which extends the entire width of the plate so as to remove heat at the maximum possible rate.

A further object is to provide an improved method of quenching steel plates by water in which the rate at which water is supplied, the pressure of the water, and the angle of the spray impingement are all related to the area impinged, according to a novel formula to produce maximum quenching effectiveness.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph with logarithmic coordinates showing data points demonstrating the manner in which the coeflicient of heat transfer between steel plates and impinging water sheets during the early part of the quench varies with a parameter, hereinafter defined, which includes the rate at which the quantity of water is applied in gallons per minute, the pressure of the water in pounds per square inch, the angle between the centerline of the water stream and the surface of the plate and the area, in square feet, of the surface of the plate being impinged by the stream of water.

FIG. 2 is a graph, with logarithmic abscissas and arithmetic ordinates and showing data points enveloped by two dashed line curves, which illustrates the manner in which the yield strength of a steel plate quenched with impinging water sheets varies in accordances with said parameter. Also shown on FIG. 2 is the solid line curve as replotted from FIG. 1.

FIG. 3 is an isometric drawing, partly in section, illustrating the water quenching of a steel plate in the manner of this invention by the impingement of water against the top and bottom surfaces of a heated steel plate being moved out of a furnace.

A novel feature of this invention is that the area impinged by the water sprays extends completely across the surface of the plate without any gaps upon the surface of the plate between individual sprays.

In carrying out the method of this invention, a steel plate to be hardened is heated to a hardening temperature at which it is fully austenitic and then quenched from a temperature at which it is fully austenitic by moving the heated plate through sprays of water having certain characteristics, hereinafter described.

The sprays of water of this invention are impinged on the plate during the initial period of the quench for a time long enough to minimize the formation of high temperature transformation on products. The required time will vary with the composition and thickness of the plate being quenched. As is well known in the art, guidance in determining this time can be obtained from isothermal time-temperature-transformation diagrams and from continuous cooling transformation diagrams characteristic of the specific composition. Following this initial period of quenchng, the plate may, if desired, be subjected to additional quenching action.

Applicants have discovered that by relating a water quench to the area of the surface of a steel plate in the process of being hardened they have found an unexpected critical value for the effectiveness of the quench.

In carrying out this invention, the quantity and the pressure of the water and the angle of impingement are selected so that the parameter Q /P sin 6 during said initial period, is not less Lhan a critical value of about 600. In the parameter Q /P sin 0, Q is the rate at which the water is supplied, in gallons per minute per square foot of surface impinged; P is the gage pressure of the water at the nozzle, in pounds per square inch; and 0, the impingement angle, is the angle between the surface of the plate and the centerline of the water. It has been determined that for effective quenching the factors Q and P cannot be less than certain minimum values. Q must be high enough, not less than about 30, so that the water carries away the heat from the plate without becoming excessively hot, and P must be high enough, not less than about 35.

So far as the applicants are aware, there has been no prior appreciation of the fact that, as the pressure of the quench water is increased, the flow rate of this water can be decreased provided Qx/P sin is maintained at a value not less than the critical value of about 600. Increasing the pressure without altering the other variables will result in increasing the rate of the quench water directly contrary to the applicants teaching. In other words, applicants have discovered that the rate of heat transfer from the plate to the quench water does not continue to increase as pressure and flow rate (i.e., gallons of quench water per minute) are increased but, on the contrary, levels off as Q /P sin 0 reaches a critical value of about 600.

The water impinges on only a portion of the area of the plate at any one time as the plate is moved through the quench. There is thus obtained progressive quenching of the entire plate. A preferred method of accomplishing this is to move the plate through a quenching zone having a width at least equal to the dimension of the plate in the direction transverse to the direction of movement, and a length preferably five or more, times the thickness of the plate. This preferred quenching zone is equivalent to a relatively narrow continuous transverse band. The length of the zone minimizes longitudinal flow of heat within the plate from the hot to the quenched portion. The nozzles are arranged so that the quench water impinges upon the entire area of the quenching zone. Any gaps in the area impinged by the quench would cause marked differences in the hardness and mechanical properties of the impinged areas as compared with those of the gap areas. Overlap of the impinged areas is wasteful of water.

As mentioned before, the high intensity of this invention spray is impinged on the plate for a time long enough to minimize the formation of high temperature transformation products, the required time depending on the composition and thickness of the plate being quenched. To illustrate cooling rates obtainable by quenching with this invention, sprays applied to opposite sides of a plate of carbon steel initially at a uniform temperature throughout of 1660 F. will cool the center of the plate to approximately 500 F. in 6 seconds if the plate is A" thick and in 16 seconds if it is thick. Quenching time, when the plate is moved relative to the sprays, is determined by the speed of the plate and the length of the quenching zones. For examples, if the plate moves through a quenching zone one foot long and having a width at least equal to the width of the plate at a speed of 10 feet per minute, the length of time that each portion of the plate is in the quenching zone would be 6 seconds. Longer quench time can be obtained by making the quenching zone longer and/or by moving the plate at a slower speed.

When quenching plates in accordance with this invention the quench water is impinged simultaneously on opposite sides of the plate to minimize Warping. In some instances it may be desirable to quench only one side of the plate.

Devices having holes or slots may be used together with deflectors to spread the water into sprays or sheets. As previously noted, such sheets are to impinge upon the plate, on opposite sides thereof, over uninterrupted areas extending across the plate as distinguished from jets and sprays which impinge upon the plate in discrete areas.

Preferably the initial quenching herein described takes place as close as possible to the exit of the furnace to minimize heat loss before quenching begins. The sheets of water are directed at opposite sides of the plate in the direction in which the plate is withdrawn from the furnace to prevent the water from flowing into the furnace. Thus the angle of impingement is preferably less than In FIG. 1, the data points and the solid curve 16 show the manner in which the initial heat transfer coefficient h in B.t.u./ft. hr. F. between a steel plate and sheets of water is dependent on Q /P sin 0. FIG. 1 shows clearly that it increases to a maximum as Q /P sin 0 is increased to a value of about 600 and remains constant at that maximum as Q /P sin 0 is further increased to values above about 600.

FIG. 2 shows data points which illustrate the manner in which the yield strength of a steel plate varies With the value of Q /P sin 0. The data points are enveloped by dashed line curve 17, representing the maximum of the range of data, and dashed line curve 18, representing the minimum. All of the yield strength data were obtained from tensile tests cut from A" thick plates of a heat of steel having the composition shown in Table I after quenching from a temperature above the Ac with sprays impinging on opposite sides of the plate, at an angle 90, and tempering at 800 F.

The curve 16 of FIG. 1, replotted to the diiferent ordinate scale, is also shown on FIG. 2 as the solid curve 16. The similarity of the shape of curve 16 and the shape of the envelope formed by curves 17 and 18 is notable. However, attention is directed to the large scatter in the yield strength data points below a value of about 600 for Q /P sin and especially at intermediate values of Q /P sin 0; this scatter will be referred to later. FIG. 2 shows clearly that the maximum cooling rate possible by water quenching as herein described is achieved by maintaining the quantity and pressure of the water, the impingement angle and the area impinged so that the value of Q /P sin 0 is not less than about 600 and that such cooling rate results in a yield strength that equals or exceeds the yield strength obtained with a specimen of the same steel by means of a violently agitated submerged water quench and an 800 F. temper, as indicated by the horizontal dashed line 19 at a value of 122,000 p.s.i.

It is apparent that in water quenching according to this invention, increasing the value of Qx/P sin 6 above the critical value of 600 will not increase the rate of cooling of the plate. This invention thus teaches not only how to obtain the maximum cooling rate in water quenching but also how to obtain this rate with the least consumption of both water and energy required for pumping it.

Another noteworthy practical point is that at values of QVF sin 0 appreciably less than 600 there is not only the possibility of obtaining less than elfective quenching but also the danger of obtaining variable or unpredictable properties of the plate because of changes in h and therefor in cooling rate accompanying small and sometimes unavoidable changes in Q and/or P. The latter point is illustrated in FIG. 2 by the scatter in the yield strength data at values of Qx/P sin 0 below 600.

It is of interest that the curve of h vs. Q /P sin 0 of FIG. 1 is steepest at or near a value of Qx/P sin 0 of 177, which is its value when Q and P are both at their respective minimum values of 30 and 35. The fact that in this range the value of h is most sensitive to changes in Q /P sin 0 suggests that the transfer of heat from plate to water is changing from one mechanism to another. This, together with the greater scatter in the data for h in and below this range, again points to the necessity, for a given value of 6, of maintaining Q at or above its specified minimum value and adjusting P to aprovide a critical value of not less than about 600 for Q /P sin 0 or of maintaining P at or above its specified minimum value and adjusting Q to provide a critical value of not less than about 600 for Q /P sin 0. In other words, maximum cooling rates cannot be obtained by quenching at too low a pressure even with a very large quantity of water nor can they be obtained if too little water is supplied even at very high pressure. It should also be noted that Q /P sin 0 decreases as 0 decreases.

In FIG. 3 there is shown plate 10, having a thickness 2, being quenched by the method of this invention. Plate 10 discharging from furnace 11 passe through quenching apparatus 12, which is located as close as convenient to the exit end of the furnace. Quenching apparatus 12 includes upper water main 13 positioned above plate 10 and lower water main 13' positioned vertically beneath upper main 13 and under plate 10. Water mains 13 and 13' extend transversely of the longitudinal centerline of plate 10 moving from furnace 11, and both mains have a length L which is equal to the width of the widest plate that can pass through furnace 11. Closely spaced along the bottom of water main 13, for its full length, are a plurality of openings 14. Extending downwardly from water main 13, for its full length is curved deflector plate 15 which is secured at edge 16 to the bottom of main 13 adjacent openings 14, on the furnace side thereof. Opposite edge 16 of deflector plate 15 is discharge edge 17, and between these two edges is curved flow control surface 18. Lower water main 13' is constructed in similar fashion except that nozzle openings 14' are located in the top of the main, and deflector plate 15 is secured at edge 16 to the top of main 13' adjacent openings 14', on the furnace side thereof. Opposite edge 16 of deflector plate 15' is discharge edge 17 and between these two edges is curved flow control surface 18'.

Water under high pressure, from a source not shown, emerges from openings 14 of main 13 as multiple sprays 19. Deflector plate 15 is positioned to intercept the flow paths of sprays 19 and cause them to converge upon curved flow control surface 18 and to be discharged from the discharge edge 17 of deflector plate 15 as a continuous spray or sheet of water 20, the cross-section of which is illustrated by dotted lines x-y. Water sheet 20 directly impinges the area of quench zone 21 bounded by the two sides of plate 10 and dotted lines A and B. Quench zone 21 has a length 1 which is about five times the thickness t of plate 10. The centerline, designated 22, of water sheet 20 forms an angle of about 22 with the top of plate 10. This angle is of course controlled by the position of discharge edge 17 of deflector plate 15. In like fashion water under high pressure from a source not shown emerges from openings 14' of main 13' as multiple sprays 19. Deflector plate 15 is positioned to intercept the flow paths of sprays 19 and cause them to converge upon curved flow control surface 18' and to be discharged from the discharge edge 17' of deflector plate 15 as a continuous spray or sheet of water 20, the crosssection of which is illustrated by dotted lines x'y'. Water sheet 20' directly impinges upon a quench zone, not shown, which is similar to zone 21 shown for the upper side of plate 10. The lower quench zone, not shown, has a length which is about five times the thickness t of plate 10. The centerline, not shown, of water sheet 20' forms an angle of about 22 with the bottom of plate 10. FIG. 3 illustrates the quench zone 21 at a particular time interval and it is recognized that the leading portion 23 of plate 10 has already been quenched and that the trailing portion 24 of the plate will pass through the quench zone and be quenched.

Table II gives examples for combinations of water flow and pressure yielding a value of 600 for QVF sin 0 and illustrates how water flow and pressure may be varied within wide limits while maintaining the maximum possible cooling rate.

Following the initial high intensity quench, further quenching may be carried out with streams with a value for Q /F sin 6 of less than 600.

The present invention is applicable to the hardening of carbon steel plates and, for the first time, makes it possible by water quenching to produce effectively hardened carbon steel plates in thicknesses which heretofore required alloy steel. Moreover, the invention is applicable to the quenching of alloy steel plates, making it possible to decrease the alloy content necessary for eifectively hardening a plate of given thickness or to harden a thicker plate to given alloy content, or both.

We claim:

1. A method of hardening a steel plate by water quenching to maximize the hardening effect under minimum optimum quenching parameters, said method comprising:

(a) heating the steel plate to a temperature not less than the Ac temperature,

(b) quenching the heated plate by moving it through a quench zone formed by continuous sheets of water arranged to directly impinge the entire width of both sides and a portion of the length of the plate,

(c) said heating and said quenching of the plate are performed sequentially on successive transverse portions of the length of the plate,

(d) correlating the rate, pressure an angle of impingement of the quenching water such that the product of Q P sin A is about 600 to maximize the heat transfer coefiicient, wherein Q is the rate at which water is supplied in gallons per minute and not less than 30 g.p.m.,

P is the gage pressure of the water in pounds per square inch and not less than 35 psi,

0 is the angle formed at the intersection of the centerline of the sheet of water and the surface of the plate,

A is the area, in square feet, determined by the product of the entire width of the plate times 8 the portion of the length of the plate directly impinged by the sheet of water, and (e) moving the heated plate through the quench zone at a rate which will minimize the formation of high temperature transformation products.

References Cited UNITED STATES PATENTS 2,776,230 1/1957 Scott 148-153 3,036,825 5/1962 Eisenmenger 148-157 X 3,294,599 12/1966 Huseby 148-143 2,395,184 2/1946 Hume et a1 148-153 X 3,289,449 12/ 1966 OBrien 266-6 S 3,300,198 1/1967 Clumpner et al 266-6 S 3,367,804 2/1968 Teplitz 148-157 X 3,420,083 1/ 1969 Satford et a1 72-201 3,479,853 11/1969 Berry 266-6 S 3,531,334 9/1970 Schrader et a1 148-157 X. 3,546,911 12/1970 Lanz 148-157 X OTHER REFERENCES French, Quenching of Steels, Amer. Soc. for Steel Treating, Cleveland, Ohio, 1930, pp. 106-112.

CHARLES N. LOVELL, Primary Examiner US. Cl. X,R. 148157 

